1. Field of the Invention
The present invention relates generally to optical devices and prisms, and, more particularly, to separating a white light beam into several colored light beams.
2. Description of the Related Art
FIG. 1 illustrates a color-separating prism based on a cross cube 10. The cross cube 10 has a square cross section and is composed of four glass prisms 12, 14, 16, 18. A first reflective layer 20 lies along a first principal diagonal of the cross cube 10. A second reflective layer 22 lies along a second principal diagonal, and intersects the first layer 20 at a right angle. The first and second reflective layers 20, 22 are multi-layers of dichroic material that selectively reflect certain wavelength or color ranges of light.
Referring to FIG. 1, a white light beam 24 enters the cross cube 10 through a first face 26 and is selectively reflected by the first and second reflective layers 20, 22. The first reflective layer 20 reflects red light 28 from the incoming white light beam 24 through a second face 30 of the cross cube 10. The second reflective layer 22 reflects blue light 32 from the incoming white light beam 24 through a third face 34 of the cross cube 10. Green light is not substantially reflected by either the first or second reflective layers 20, 22. Therefore, the green light 36 from the incoming white light beam 24 is transmitted through a fourth face 38 without substantial deviation. The cross cube 10, therefore, separates the incoming white light beam 24 into separate red, blue and green light beams 28, 36, 32, respectively, going in different directions.
FIG. 2 illustrates an exemplary percent reflectivity of the first and second reflective layers 20, 22 of the cross cube 10 in FIG. 1 as a function of wavelength in nanometers (nm). A solid line 39 shows exemplary values for the reflectivity of the first reflective layer 20. The first layer 20 reflects substantially all visible red light and some infrared light at wavelengths greater than about 625 nm. At wavelengths below about 600 nm, the first reflective layer 20 is substantially transparent to visible light. A broken line 40 shows exemplary values for the reflectivity of the second reflective layer 22. The second reflective layer 22 reflects substantially all visible blue light below wavelengths of about 460 nm. Above a wavelength of about 460 nm the second reflective layer 22 is substantially transparent to visible light. The reflectivities of the different dichroic materials making up the first and second layers 20, 22, give the cross cube 10 in FIG. 1 its color-separating properties.
FIG. 3 illustrates a second color-separating prism that is generally referred to as a Philips prism 42. The glass elements of the Philips prism 42 include first and second component prisms 44 and 46, and a cover element 48. A first reflective layer 50 is deposited on a back surface 52 of the first component prism 44. A second reflective layer 54 is deposited between a back surface 56 of the second component prism 46 and the cover element 48. Mountings 58, 60 rigidly position the first and second component prisms 44, 46 with respect to each other so that an air gap 62 exists between the first reflective layer 50 and a front surface 66 of the second component prism 46. The Philips prism 42 separates an incoming white light beam 68 into color components, because the first and second reflective layers 50, 54 selectively reflect blue and red light, respectively. The order of light (e.g., red first, then blue or vice versa) is changeable by changing the layers 50 and 54.
The advantage of the Philips prism 42 over the cross cube 10 is that the incident angles of an incoming light ray to the prism interfaces are less steep. Thin-film coaters can optimize the coatings to get better performance than in the cross cube configuration. Referring to FIG. 3, the incoming white light beam 68 passes through a front surface 70 of the first component prism 44. The first reflective layer 50 is constructed of layered dichroic material (e.g., thin film coating) as is the second reflective layer 22 in FIG. 1. An incoming ray of blue light 72 is reflected by the first reflective layer 50 back towards the front surface 70 of the first component prism 44. If the blue light 72 (e.g., a chief ray of a core of light) is substantially perpendicularly incident on the front surface 70, reflected blue light 74 is re-incident on the front surface 70 at an angle that is greater than the critical angle for total internal reflection. Then, the reflected blue light 74 is reflected by the front surface 70 as blue light 92 toward a third surface 76 of the first component prism 44. An incoming ray of red light 78 passes through the first reflective layer 50 without being substantially reflected. The ray of red light 78 is, however, reflected by the second reflective layer 54. If the ray of red light 78 (e.g., collimated light or a chief ray of a core of light) is substantially perpendicularly incident on the front surface 70 of the first component prism 44, a reflected ray of red light 80 is re-incident on the first surface 66 of the second component prism 46 at an angle that is greater than the angle for total internal reflection. Then, the reflected ray of red light 80 is reflected as red light 88 toward a second surface 82 of the second component prism 46. An incoming ray of green light 84 incident on the front surface 70 of the first component prism 44 passes through the first and second reflective layers 50, 54 substantially undeviated. The ray of green light 84 is transmitted through a back surface 86 of the cover element 48 as green light 90. The Philips prism 42 thus separates the incoming white light beam 68 into the red light 88, the green light 90 and the blue light 92, all traveling in different directions.
Referring to FIGS. 1 and 3, the cross cube 10 and the Philips prism 42, respectively, have several inconvenient properties. In the cross cube 10, the first and second layers 20, 22 make 45.degree. angles with respect to the first surface 26. The 45.degree. arrangement simplifies the construction of the cross cube 10, but may make the cross prism 10 inconveniently thick. Also, the reflectivities and transmissivities of the first and second layers 20, 22 may differ for the two polarizations of the incoming white light beam 24, because the light beam 24 is not perpendicularly incident on the first and second reflective layers 20, 22. The reflectivities are often polarization-dependent for non-perpendicular incidence. Further, though the Philips prism 42 in FIG. 3 has less polarization-dependent reflectivities, due to the more perpendicular incidence of the white light beam 68 on the first and second reflective layers 52, 54, this same arrangement may also make the Philips prism inconveniently thick. Moreover, for the cross cube 10, another disadvantage is that the center of the "X" may be projected (e.g., in a projection system) to a screen and seen as a line to a viewer of the screen.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.